For industrial catalysts, automobile exhaust gas purifying catalysts and many electric appliances, precious metals such as palladium, platinum and rhodium have been used. Since precious metals are expensive and useful as resources, they have been recovered after use and recycled. In recent years, demands for the resources conservation are increasing, and importance of separation and recovery, and recycle of precious metals is more increasing.
To separate and recover a precious metal, many methods such as a sedimentation separation method, an ion exchange method, an electrodeposition method and a solvent extraction method have been developed, and among them, a solvent extraction method has been widely employed in view of the economical efficiency and operation properties.
A solvent extraction method is one of methods to separate and recovery a precious metal. Specifically, it comprises an extracting step of subjecting an aqueous phase in which palladium ions are dissolved and an organic phase in which an oil-soluble extracting agent is dissolved to liquid-liquid contact to extract the palladium ions into the organic phase side, and a back extraction step of back-extracting the palladium ions which had been extracted to the organic phase side into the aqueous phase side again using an aqueous phase in which a back-extracting agent is dissolved. For example, extraction of palladium ions is carried out with an organic phase using a dialkyl sulfide compound as the extracting agent, and back extraction of palladium ions is carried out with an aqueous phase using an aqueous ammonia solution as the back-extracting agent (for example, Patent Document 1).
Further, in order to improve the extraction rate in the solvent extraction method, an extracting agent having an amide group introduced to the vicinity of sulfur in the dialkyl sulfide has been proposed, and a method for back-extracting palladium using an acidic thiourea aqueous solution as the back-extracting agent has been proposed (for example, Patent Document 2).
However, the above-described solvent extraction method has problems in view of the safety and the environmental burden, since a large quantity of an organic solvent is used.
Accordingly, as a method using no organic solvent, a method of adsorbing palladium ions has been proposed (for example, Patent document 3). In this method, as a ligand (receptor) of palladium ions, a specific sulfide compound is fixed on a styrene derivative polymer (for example, polychloromethylstyrene) carrier to prepare an insoluble palladium ion adsorbent, and this adsorbent is directly added to an aqueous solution in which palladium ions are dissolved to carry out adsorption of the palladium ions.
Further, the applicant has already been filed a patent application relating to an adsorbent comprising a compound having an amide group introduced to the vicinity of sulfur in a dialkyl sulfide as a ligand (for example, Patent Documents 4 and 5).
In a case where an extracting agent having an amide group introduced to the vicinity of sulfur in a dialkyl sulfide or an adsorbent comprising a compound having an amide group introduced to the vicinity of sulfur in a dialkyl sulfide as a ligand, is used, the back extraction rate or the desorption rate of palladium ions tends to be low if hydrochloric acid or ammonia is used as a back-extracting agent or desorbent, and accordingly usually a thiourea aqueous solution is preferably used (for example, Patent Document 5).